1. Field of the Invention
This invention relates to the production of methanol from offshore natural gas by processing the natural gas in a plant erected on a floating structure.
Crude oil exploration is taking place throughout the world in offshore areas, and in many cases gas is found associated with oil but in quantities too small to be economically transported to shore. This gas must therefore either be wastefully burned or returned to the formation at considerable expense.
Several approaches have been suggested for utilizing offshore gas where it occurs in amounts unsuitable for pipelining to shore. One such method involves liquefaction of the natural gas followed by transportation of the liquefied natural gas to an onshore processing facility. This approach is attractive in some respects, but is subject to considerable economic and processing limitations. Another approach which has been suggested is to produce ammonia or urea on a floating structure. The other alternative which has been considered by the industry is construction of a floating methanol plant for use at offshore gas sites.
2. Prior Art
As mentioned above, the broad idea of constructing a processing plant on a floating structure for use at an offshore gas site is well known. A general discussion of the alternatives and the advantages and disadvantages of various approaches appears in the Nov. 24, 1976 issue of Chemical Week. As noted in that article, the problems involved are of such a magnitude that there are no existing facilities for processing natural gas on a floating structure.
An ice plant constructed on a ship is described in U.S. Pat. No. 2,630,091, and U.S. Pat. No. 3,837,308 describes a floating power plant.
Plants for the production of methanol using natural gas as a feedstock are well known. These plants consist of a feed gas pre-treatment plant, a steam-natural gas reformer, a steam-producing waste heat recovery system usually combined with the exhaust of the reformer, a synthesis reactor and a distillation section. In these plants, a large amount of heat is produced in the synthesis reactor, and unless a large portion of this heat is recovered, then the process will be very inefficient. Many recent methanol plants utilize a basic design developed by Imperial Chemicals Industries and known generally as the ICI low-pressure methanol process. This process is described in the January 1975 issue of Hydrocarbon Processing. The Hydrocarbon Processing article also describes variations in which the heat for the reboiler of the distillation columns is obtained from the reformed gas as an energy conservation measure. Plants utilizing the ICI design are presently available, and the basic process of converting natural gas to methanol does not constitute a part of this invention.
A processing plant located on a floating structure is subject to motion from wave action and weather conditions, and certain types of process equipment are more sensitive to motion than others. In particular, tall distillation columns are extremely sensitive to motion, as the efficiency of the columns drops drastically if the columns deviate more than a very few degrees from the vertical, and desalination units which are necessary for production of fresh process water when the plant is located in sea water are sensitive to motion, and must be designed somewhat oversize and fresh water storage means provided to allow the plant to operate using stored fresh water during severe weather. On the other hand, the other sections of a methanol plant using natural gas as a feedstock are not particularly sensitive to motion, and can be operated more or less normally even in reasonably severe weather conditions on a floating structure. Accordingly, a conventional methanol plant cannot be effectively used on a floating structure because of the sensitivity of the distillation section and desalination unit to motion, and modifications are necessary in order to provide an efficient plant on a floating structure.
It is an object of the present invention to provide an efficient and flexible plant for the production of methanol from offshore natural gas on a floating platform.
It is a further object to provide such a plant wherein the distillation section and desalination unit are independent of the rest of the plant such that they can be shut down during severe weather without the necessity for shutting down the entire plant.